Forensic Engineering 7th Congress
Performance of the Built Environment
November 15 – 18, 2015
Hilton Miami Downtown Hotel
Miami, FL
A New Structural and Metallurgical Review of the Great Boston Molasses Tank
Failure of 1919
Ronald A. Mayville1
Simpson Gumpertz & Heger Inc., 41 Seyon Street, Waltham, MA, 02453; PH (781)
907-9414; FAX (781) 907-9000; e-mail: [email protected]
1
ABSTRACT
The Boston molasses tank failure is a famous incident in which 21 people lost their
lives. The cause of the failure was never definitively determined even though the
court auditor was presented with over 20,000 pages of conflicting testimony by
prominent engineering experts from institutions like MIT and Harvard. The author
has examined evidence presented in the case as well as private failure analysis reports
by experts of the day in light of our current understanding of structural design,
analysis methods, and metallurgical effects. There were no applicable structural
codes when the tank was designed, but the practice for designing riveted structures
was documented in well-known text books. The author has conducted finite element
analysis to calculate the stress and strain conditions that existed just prior to failure as
well as to simulate the tank collapse and flow of molasses against adjacent structures,
many of which were destroyed. The case is also examined with respect to the
methods we currently use to protect against brittle fracture, including the use of
toughness tests and the effects of steel chemistry. The work includes discussion on
the differences between the structural steel used to fabricate the tank and what we use
today, including an interesting reference to the Titanic failure.
INTRODUCTION
Great structural failures have occurred throughout history and it is human nature to
try to understand why they occurred for both the betterment of society and to
determine who is at fault. The great Boston molasses failure of 1919 is a particularly
interesting case because of the number of fatalities (21) and the curious manner in
which people perished. Nearly two and a half million gallons of thick liquid
suddenly and without resistance flowed as a great wave into the streets of Boston
leveling buildings like a tsunami and nearly collapsing the adjacent elevated railway.
For decades after the accident, residents claimed they could smell of molasses on hot
days. An account of the social aspects of the tank failure has been provided by Puleo
[1].
The failure led to a four-year long hearing that produced 25,000 pages of testimony
supported by many experts from leading institutions of the time like MIT and
Harvard. Numerous tests were conducted by the experts including the fabrication of
a sub-scale tank in which a bomb was exploded in an effort to demonstrate the
similarities between the outcome of that test and the actual tank failure.
This paper presents the results of a fresh look at the tank failure using some of the
forensic tools we have available to us today that they did not have then. In particular,
we use both finite element analysis and fracture mechanics to evaluate the conditions
under which the failure occurred. The author has had a long interest in the tank
failure stemming from an early fracture mechanics class and fueled by residence
within a few miles of the original tank site.
Figure 1. The molasses tank in the background with the elevated railway to the
left. The buildings shown were destroyed in the accident. (Photo: Courtesy of
Bill Noonan, Boston Fire Department Archives.)
The tank ruptured at about noon on January 15, 1919, destroying the buildings shown
in Figure 1 and damaging the elevated railway, Figure 2. The temperature at the time
of failure, and for several hours prior to the failure, was about 40°F. There was no
snow on the tank and the reported wind speed was about 15 mph. The tank had been
filled to a level of 2.3 million gallons just two days prior to the failure, but had
previously experienced about 30 substantial changes in the fill level. The tank broke
into several pieces including a four ton piece of the bottom ring 1 that included half
of the manway and part of the tank bottom. This piece was found about 200 ft from
the original tank location (in the foreground of Figure 1) and at least one expert
testified its position was the result of a parabolic trajectory resulting from an
explosion within the tank.
Figure 3. Damage to the adjacent elevated railway from the molasses tank
failure. (Photo: Courtesy of Bill Noonan, Boston Fire Department Archives.)
TANK DESCRIPTION
The molasses tank was owned by U.S. Industrial Alcohol (USIA) who converted the
molasses into alcohol for the production of munitions to support the First World War.
The tank was designed and constructed by the Hammond Iron Works of Warren,
Pennsylvania. Most of the information about the tank comes from the testimony of
experts during the hearing. It was 90ft in diameter and 50ft tall and consisted of
seven courses or rings of open hearth steel plate; the bottom ring had a thickness of
0.67 in. Horizontal lap joints had a single row of rivets. The bottom ring utilized a
butt joint with inner and outer cover plates and three rows of rivets on each side. The
vertical joints of the upper rings used fewer rivets. There were no industrial design
codes for tanks at that time, but the City of Boston building code placed limits on the
maximum stress in ‘structures’ that depended on the material and the potential failure
mode. Figure 4 lists the values applicable to the steel of the tank.
Figure 4. An excerpt from the Boston Building Law applicable in 1919
The tank included a single manway located in the first ring. It was riveted to the tank
and included a flange, but only on the exterior. In addition, the lap joint of this flange
was located at the top of the manway, which as our finite element analysis has shown
(see below) coincided with the maximum stress around the manway cut out. Figure 5
shows a photograph of a replica of the tank manway that was fabricated by one of the
experts, Albert Ladd Colby, for a large-scale test at Lehigh University. (The machine
in which it was tested in 1920 still stands at the university.)
Rivet holes were made in the field at the site of the tank construction by punching and
the punched holes were not subsequently reamed. The consequences of such a
fabrication method were studied extensively even in that time, primarily because of
the associated degradation in strength that occurred in boiler construction. Punching
alone could either reduce the ductility of the material adjacent to the hole or cause
cracks or both.
Figure 5. Replica of the molasses tank manway construction (Photo: Courtesy
of Lehigh University)
FRACTURE AND MATERIAL EVIDENCE
The pieces of the failed tank were analyzed and tested extensively by the various
experts and results were reported in court testimony. Notable points include:
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The tank broke into several large pieces, with one large section pushed West
towards the elevated railway and others pushed North and East of the original
location.
A four ton piece that included part of the manway and a pie-shaped section of
the tank bottom was found about 200 ft South in the playground shown in
Figure 1.
Vertically oriented fractures occurred both above the manway, at the 12
o’clock position, and below the manway, at the 6 o’clock position. The
corresponding fracture surfaces showed herringbone features that pointed to
the manway as the fracture origin for both the upper and lower fractures.
Tensile properties showed the tank steel to be of good quality of the day: yield
strength, about 35ksi, tensile strength, 56 ksi, elongations in 8 in. over 20%,
and reductions in area exceeding 50%.
Metallographic sections taken immediately adjacent to several of the fractures
revealed Neumann bands (what we now refer to as deformation twins.)
The last point is a particularly interesting one because several experts for the defense
(USIA) used such evidence to declare that an explosion within the tank was
responsible for the failure. Considerable research had been conducted as part of
military developments to support this statement. Nevertheless, we know now that
Neumann bands can be produced by the very rapid crack propagation that occurs in
fracture of brittle steels [2].
STRESS ANALYSIS
At the time the tank was designed, stress analysis was conducted only by hand.
Design formulas were available from text books for riveted joints but the concept of
stress concentration factors was apparently not used in analysis of structures. The
maximum nominal hoop stress in the tank, considering the head pressure of the
molasses at the time of failure but without consideration of corner or lap joint
reinforcement, was 28.7 ksi. The calculated bearing and rivet shear stresses for the
vertical diamond pattern butt joint in the first ring were 15.4 and 32.4, respectively
[3]. Each of these three stress quantities exceeds the stress limits of the Boston
Building Code (Figure 4) by a large amount: 55 to 80% depending on the type of
stress. It is a mystery to the author how the designers could make such a large
mistake. Perhaps the they thought the tank would be filled with water, which is 40%
lighter than molasses.
We have now conducted finite element analysis of the region of rivet holes adjacent
to the manhole using the computer program Abaqus [4]. The stress-strain curve for
the tank steel was estimated from the tensile properties and we did not consider the
possible effect of work hardening from the riveting process. The results show that
the most highly stressed area, which is the rivet hole directly above the manhole,
likely experienced plastic deformation; Figure 6.
Figure 6. Contours of calculated plastic strain at the rivet holes of the molasses
tank corresponding to the molasses level at the time of failure.
Fracture mechanics can be used to assess the severity of a punching-induced crack
that may have existed at the edge of one of the manway rivet holes. Such an analysis
is complicated by the nonuniform stress distribution in the vicinity of the manway
and the fact that there was plastic deformation, but one can use linear elastic fracture
mechanics to approximately assess the risk of crack growth. The stress intensity for a
small crack at the edge of a hole under uniform tension is given by the equation [5]:
𝐾𝐼 = 𝐹𝜎√𝜋𝑎
where F = 3.4. We do not know the fracture toughness of the molasses tank steel, but
we can estimate it from the data for a comparable steel of the time. The Titanic was
fabricated from such steel and after discovery of this structure in 1986 material was
recovered and tested [6]. We estimate the toughness at 40°F, the temperature at the
time of the accident, to be 30 ksi√in. Using this value with a nominal stress of 25 ksi
gives a critical crack size of 0.05 in., about 5% of the hole diameter. Again, this is
not an accurate calculation because of the presence of plastic deformation and
because the thickness of the plate is less than required to provide plain strain
conditions.
We have also conducted finite element analysis to simulate the actual failure of the
tank. The approach uses the Eulerian methods of Abaqus and includes the fluid
properties of the molasses and the deformable properties of the tank. The current
model simplifies the fracture as having occurred in a few specific locations. Figure 7
shows an image at approximately two seconds after initial failure and even shows
how the four ton, pie-shaped section could be moved by the molasses alone without
the aid of an explosion.
Figure 7. Dynamic simulation result of the molasses tank failure two seconds
after failure initiation.
DISCUSSION
The initial, public judgment of the molasses tank failure was that it arose from an
explosion. The defense in the subsequent hearing used the evidence of tank pieces
having been moved great distances with great force, the physical similarity of the
fracture patterns and the metallurgical evidence of Neumann bands in an effort to
prove that there had been an explosion. The plaintiffs could easily show that the tank
was underdesigned and were able to find alternate explanations for the evidence on
which the defense relied. The auditor ruled in favor of the plaintiffs and awarded
them $300,000 (about $4 million in 2014 dollars.)
Our results support the conclusion of the auditor. We have shown that the stresses
and strains around the rivet hole at the top of the manway were severe. Our estimate
of the stress intensity factor for a crack at this location indicates that a pre-existing
crack from the rivet hole punching process could have been sharpened by the 30
significant cycles of tank filling eventually leading to the initiation of rapid fracture.
Our research continues with one of the main mysteries being how a reputable firm
like the Hammond Iron Works could have made such a substantial design error.
ACKNOWLEDGMENTS
The author would like to acknowledge the assistance in searching the literature of Mr.
Albert Saul and Ms. Joan Cunningham, both of Simpson Gumpertz & Heger Inc., and
of Mr. Brian Harkins of the Social Law Library, and of Dr. Ozgur Yapar, of Dassault
Systems for assistance in conducting the dynamic finite element analysis.
REFERENCES
1.
Puleo, Stephen, Dark Tide (Boston; Beacon Press)2003
2.
Iung, T., Di Fant, M., and Pineau, A., Dynamic Fracture and Crack Arrest
Behaviour of a Pipeline Steel Investigate with a New Specimen Geometry:
The Ring Test,” in ECF9, Reliability and Structural Integrity of Advanced
Materials (1992)
3.
Mayville, R.A., “The Great Boston Molasses Tank Failure of 1919,” Civil +
Structural Engineer (Sept. 2014)
4.
Abaqus, Dassault Systems
5.
Barsom, J.M. and Rolfe, S.T., Fracture and Fatigue Control of Structures, 2nd
Edition, Prentice-Hall, NJ, 1987.
6.
Foecke, T.,”Metallurgy of the RMS Titanic,” National Institute of Standards
and Technology, Report NIST-IR 6118 (1998)18 pages.